Add with-keyword sketching.

sketching/may_2025/with.dm

@@ -0,0 +1,12 @@
+class Greeter(fld greeting: String) {
+    pub fn greet() {
+        println(greeting);
+    }
+}
+
+fn main() {
+    let greeter = Greeter('Hello, world!');
+    with greeter {
+        greet()
+    }
+}